The biggest of all the construction projects is the Google Experience Center, which is being built at the center of the Googleplex. The Experience Center will be a facility used primarily to show off Google products to potential clients, analysts and other VIP partners. The center will be 120,000 square feet and will not be open to the public, but rather kept specifically for invited guests so Google can "share visionary ideas, and explore new ways of working". Apparently, the center will be able to hold about 900 guests and will be highly flexible in order to accommodate various groups and various presentations. Overall, it should be an impressive space to help Google push products to enterprise clients and more.
In addition to the Experience Center, Google is also building a "radio frequency testing facility" just a few blocks from campus. That facility has been labeled as "Google@home", which seems to point to a testing facility for its hardware related to Android@Home, such as the stereo system that we've heard about, and possibly the "personal communication device" as well.
Google will also be modifying a lab for "Project X", but not too much is known about that.